Since the barrel solution doesn't work anymore-
Expedition- Mode: replay chapters elite, Level: The Acropolis, Difficulty: Adventure.
Head down the path towards the quest marker and into the cutscene ahead. After skipping/watching the cut scene, shoot the legs of the two guards in front of you (don't kill them). After attracting their attention, back up to the area before you triggered the cutscene and hide behind those crates on the right hand side. Stay there until eventually a little mob of guards will swarm the entryway where the cut scene happened and you can just mow them all down. Easy-Peasy
Note that this is a great level to get the other achievements below because there is a checkpoint you can reload over and over after the cutscene:
-Laying Down the Law
-Blade of Justice